And It Was All Yellow

Over the past few weeks I have spent most of my time familiarizing myself with different types of yellow dyes and pigments, learning about historic methods of paint making and sifting through the chemistry behind organic yellows. I can safely say I will never look at the color yellow the same way again. Despite the hours spent separating minute samples of these colorants, I find the engagement with color to be a fascinating and enriching process. After a background survey to determine the yellows most likely to appear in 18th century colonial oil paintings, I set out to establish a sort of database of comparative SERS spectra from known reference yellows.

After learning precisely which yellows were most frequently used and determining the chromophore, or color-causing chemical, in each, I ordered a large collection of brilliant yellows from Kremer Pigments. As my first challenge was to collect clear and accurate spectra from each dye, pigment, and chromophore we ordered, I hoped most would not give me too much trouble. Even though the terms “dye” and “pigment” are often used interchangeably, I learned that they are actually quite different. A dye refers to the basic, organic colorant such as ground up parts of a plant, while a pigment refers to this dyestuff complexed in an inorganic binding medium, such as the alum we use in lab. This inorganic binder creates a basis for many “lake” pigments, which have been used throughout history for a number of prominent paints such as carmine lake, stil de grain lake, and reseda lake. These pigments create brilliant oil paints but are classified as highly fugitive paints, or prone to light fading.

The dyes I acquired were turmeric, old fustic, saffron, gamboge, and buckthorn berries, while the pigments I ordered were stil de grain lake, reseda lake, and a mixture of the two. The lake pigments will likely require some pre-treatment method before they produce accurate spectra, so I first focused on the dyes. My goal was to get matching spectra from the chromophore and the dye in order to break down the colorant to its basic chemical properties and make sure the spectrum was accurate.

The SERS technique entails taking a tiny sample of the colorant on a glass slide and covering it with a silver nanoparticle paste. The nanoparticles make up the “surface enhanced” part of “surface enhanced Raman spectroscopy,” allowing an enhanced Raman signal to be derived from a smaller sample of organic material. The Raman signal comes from the interaction of a laser with the prepared sample, detecting energy shifts from this interaction and interpreting them as diagnostic spectra, allowing for the identification of the components of the sample. Traditional Raman spectroscopy represents a promising method of identification save for the fact that a significantly larger sample is required to produce the same peaks. The surface interaction between the silver nanoparticles and the sample enhances the traditional Raman signal, while also allowing for some fluorescent organic materials to be viewed clearly.

The spectra I derived from turmeric and its chromophore, curcumin, reacted well to the SERS technique and yielded the following matching peaks.

Saffron, buckthorn berries, and old fustic also cooperated well, the last of which is shown here matching the peaks of its chromophore, morin.

Now that I know how to identify these dyes, I will be able to recognize them if they appear in an unknown sample of paint from a historic painting. However, this was the easy part. The lake pigments and gamboge are insoluble and did not cooperate with traditional SERS. This means I have to determine methods of pre-treatment before applying the nanoparticles and taking spectra. Solubility likely causes these problems and various quantities of organic solvents and acids will be applied to the samples in an attempt to extract and solubilize the organic colorants.